This invention relates to an apparatus for automatically forming openings of different sizes at desired locations in printed circuit boards by using a plurality of drills, and more particularly it is concerned with an improvement in or relating to a mechanism of the apparatus for attaching a drill to and detaching it from a drilling unit.
Generally, printed circuit boards require many openings of different diameters to be drilled at precise locations. To this end, it is usual practice to prepare, in volume production of printed circuit boards, a plurality of types of drills necessary for forming openings of required diameters and change the drills for the drilling of different sized openings. The use of drills of a plurality of sizes increases the overall size of the drilling apparatus because of the need of space for storing the drills, and exerts great influences on the operation efficiency of drilling because drill changing is a timeconsuming operation.
To aid in efficiently drilling printed circuit boards, a proposal has been made in U.S. Pat. No. 3,973,863 to use a device for automatically changing drills. This device uses drills each of which has a collar fitted around its shank at a predetermined location to ensure the tip of the drill having a precise length necessary for drilling the printed circuit boards. A plurality of tool holders each substantially cylindrical in configuration and formed with a plurality of axial slots are supported on a movable work table of the apparatus for retaining the drills. The drills of different sizes are each inserted in one of the tool holders by bringing the collar into frictional engagement with the holder, and held in position with the shank being oriented upwardly. To attach one of such drills to the drilling unit of the apparatus for forming openings in the printed circuit boards, the drilling unit and the drill holders are first moved relative to each other to align the desired drill with the spindle of the drilling unit. Then, a collet chuck at the lower end of the spindle is opened to allow the shank of the desired drill to be inserted in the collet chuck while the spindle moves downwardly. The downward movement of the spindle continues until the collet chuck abuts against the collar of the drill, and thereafter, the collet chuck is closed to grip the drill. By attaching the drill to the spindle of the drilling unit in this way, it is possible to ensure that the distance between the forward end of the spindle of the drilling unit and the tip of the drill is kept constant each time drill changing is effected. This allows drill changing to be effected while at the same time ensuring that the tip of the drill extends a precise distance to effect proper penetration of the printed circuit boards in a drilling operation.
Meanwhile, Japanese Patent Unexamined Publication No. 64246/1984 discloses a system for mounting a plurality of cutters to a machine tool by using detachable magazines. In this system, the plurality of cutters are each received in one of the through-holes formed in each magazine of a flat plate shape, and retained by frictional engagement in the through hole. Clamp means provided with a chuck mechanism that can be opened and closed is located adjacent the work table of the machine tool to insert a cutter in the magazine and withdraw same therefrom. When it is desired to effect cutter changing by using this system, the magazine is first moved to bring the desired cutter into alignment with the clamp means. Then, the clamp means is moved upwardly to grip the cutter by its chuck and withdraw same from the magazine. After the magazine is moved away, a working unit is moved to a position above the clamp means where the working unit is aligned with the cutter gripped by the clamp means. The clamp means is moved upwardly again so as to fit the cutter to the collet of the working unit. The process described hereinabove is reversed when it is desired to insert the cutter in the magazine after being used to perform drilling. Thus, this system facilitates preparing and handling the cutters and reduces space for holding cutters in a machine tool by using at least one detachable magazine for retaining the cutters.
In the drill changing device according to the U.S. Patent described above, however, a plurality of drills are arranged horizontally in rows on the work table. This arrangement offers a disadvantage in respect of requiring more space for retaining the drills. Also, in case that the chuck did not open sufficiently widely to grip the drill or the collet were out of alignment with the drill holder, frictional drag of the collet on the shank of the drill would increase in magnitude, giving rise to the danger that the collet might grip the drill without coming into contact with the collar around the shank. If this occurred, the tip of the drill would move downwardly a distance greater than the specified one and might penetrate the work table, resulting in a reduced service life of the drill or damage of the drill.
In the system according to Japanese Patent Unexamined Publication No. 64246/1984, it is required to provide each clamp means with a chuck opening and closing mechanism. Additionally, in order that the cutter may be positively transferred, the collet of the working unit and the chuck of the clamp means have to be opened and closed positively in synchronism with each other. This would give rise to the problem that the system becomes large in size and complex in construction.